1. Field of the Invention
The present invention relates generally to cardiac stimulators and, more particularly, to dual-chamber cardiac stimulators that have an improved ability to detect tachyarrhythmias.
2. Description of the Related Art
As most people are aware, the human heart is an organ having four chambers. A septum divides the heart in half, with each half having two chambers. The upper chambers are referred to as the left and right atria, and the lower chambers are referred to as the left and right ventricles. Deoxygenated blood enters the right atrium through the pulmonary veins. Contraction of the right atrium and of the right ventricle pump the deoxygenated blood through the pulmonary arteries to the lungs where the blood is oxygenated. This oxygenated blood is carried to the left atrium by the pulmonary veins. From this cavity, the oxygenated blood passes to the left ventricle and is pumped to a large artery, the aorta, which delivers the pure blood to the other portions of the body through the various branches of the vascular system.
In the normal human heart, the sinus node (generally located near the junction of the superior vena cava and the right atrium) constitutes the primary natural pacemaker by which rhyhnic electrical excitation is developed. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers. In response to this excitation, the atria contract, pumping blood from those chambers into the respective ventricles. The impulse is transmitted to the ventricles through the atrioventricular (AV) node to cause the ventricles to contract. This action is repeated in a rhythmic cardiac cycle in which the atrial and ventricular chambers alternately contract and pump, then relax and fill. One-way valves between the atrial and ventricular chambers in the right and left sides of the heart and at the exits of the right and left ventricles prevent backflow of the blood as it moves through the heart and the circulatory system.
The sinus node is spontaneously rhythmic, and the cardiac rhythm originating from the sinus node is referred to as sinus rhythm. This capacity to produce spontaneous cardiac impulses is called rhythmicity. Some other cardiac tissues also possess this electrophysiologic property and, hence, constitute secondary natural pacemakers. However, the sinus node is the primary pacemaker because it has the fastest spontaneous rate and because the secondary pacemakers tend to be inhibited by the more rapid rate at which impulses are generated by the sinus node.
The resting rates at which sinus rhythm occurs in normal people differ from age group to age group, generally ranging between 110 and 150 beats per minute (xe2x80x9cbpmxe2x80x9d) at birth, and gradually slowing in childhood to the range between 65 and 85 bpm usually found in adults. The resting sinus rate, typically referred to simply as the xe2x80x9csinus rate,xe2x80x9d varies from one person to another and, despite the aforementioned usual adult range, is generally considered to lie anywhere between 60 and 100 bpm (the xe2x80x9csinus rate rangexe2x80x9d) for the adult population.
A number of factors may affect the sinus rate, and some of those factors may slow or accelerate the rate sufficiently to take it outside of the sinus rate range. Slow rates (below 60 bpm) are referred to as sinus bradycardia, and high rates (above 100 bpm) are referred to as sinus tachycardia. In particular, sinus tachycardia observed in healthy people arises from various factors which may include physical or emotional stress, such as exercise or excitement, consumption of beverages containing alcohol or caffeine, cigarette smoking, and the ingestion of certain drugs. The sinus tachycardia rate usually ranges between 101 and 160 bpm in adults, but has been observed at rates up to (and in infrequent instances, exceeding) 200 bpm in younger persons during strenuous exercise.
Sinus tachycardia is sometimes categorized as a cardiac arrhythmia, since it is a variation from the normal sinus rate range. Arrhythmia rates which exceed the upper end of the sinus rate range are termed tachyarrhythmias. Healthy people usually experience a gradual return to their normal sinus rate after the removal of the factors giving rise to sinus tachycardia. However, people suffering from disease may experience abnormal arrhythmias that may require special, and in some instances immediate, treatment. In this text, we typically refer to abnormally high rates that have not yet been determined to be caused by myocardial malfunction as tachycardias and to abnormally high rates that have been determined to be caused by myocardial malfunction as tachyarrhythmias.
It should also be appreciated that an abnormal tachyarrhythmias may initiate fibrillation. Fibrillation is a tachyarrhythmia characterized by the commencement of completely uncoordinated random contractions by sections of conductive cardiac tissue of the affected chamber, quickly resulting in a complete loss of synchronous contraction of the overall mass of tissue and a consequent loss of the blood-pumping capability of that chamber.
In addition to rhythmicity, other electrophysiologic properties of the heart include excitability and conductivity. Excitability, which is the property of cardiac tissue to respond to a stimulus, varies with the different periods of the cardiac cycle. As one example, the cardiac tissue is not able to respond to a stimulus during the absolute refractory phase of the refractory period, which is approximately the interval of contraction from the start of the QRS complex to the commencement of the T wave of the electrocardiogram. As another example, the cardiac tissue exhibits a lower than usual response during another portion of the refractory period constituting the initial part of the relative refractory phase, which is coincident with the T wave. Also, the excitability of the various portions of the cardiac tissue differs according to the degree of refractoriness of the tissue.
Similarly, the different portions of the heart vary significantly in conductivity, which is a related electrophysiologic property of cardiac tissue that determines the speed with which cardiac impulses are transmitted. For example, ventricular tissue and atrial tissue are more conductive than AV junction tissue. The longer refractory phase and slower conductivity of the AV junction tissue give it a significant natural protective function, as described in more detail later.
For a variety of reasons, a person""s heart may not function properly and, thus, endanger the person""s well-being. Most typically, heart disease affects the rhythmicity of the organ, but it may also affect the excitability and/or conductivity of the cardiac tissue as well. As most people are aware, medical devices have been developed to facilitate heart function in such situations. For instance, if a person""s heart does not beat properly, a cardiac stimulator may be used to provide relief. A cardiac stimulator is a medical device that delivers electrical stimulation to a patient""s heart. A cardiac stimulator generally includes a pulse generator for creating electrical stimulation pulses and a conductive lead for delivering these electrical stimulation pulses to the designated portion of the heart. As described in more detail below, cardiac stimulators generally supply electrical pulses to the heart to keep the heart beating at a desired rate, although they may supply a relatively larger electrical pulse to the heart to help the heart recover from fibrillation.
Early pacemakers were devised to treat bradycardia. These pacemakers did not monitor the condition of the heart. Rather, early pacemakers simply provided stimulation pulses at a fixed rate and, thus, kept the heart beating at that fixed rate. However, it was found that pacemakers of this type used an inordinate amount of energy due to the constant pulse production. Even the sinus node of a heart in need of a pacemaker often provides suitable rhythmic stimulation occasionally. Accordingly, if a heart, even for a short period, is able to beat on its own, providing an electrical stimulation pulse using a pacemaker wastes the pacemaker""s energy.
To address this problem, pacemakers were subsequently designed to monitor the heart and to provide stimulation pulses only when necessary. These pacemakers were referred to as xe2x80x9cdemandxe2x80x9d pacemakers because they provided stimulation only when the heart demanded stimulation. If a demand pacemaker detected a natural heartbeat within a prescribed period of time, typically referred to as the xe2x80x9cescape intervalxe2x80x9d, the pacemaker provided no stimulation pulse. Because monitoring uses much less power than generating stimulation pulses, the demand pacemakers took a large step toward conserving the limited energy contained in the pacemaker""s battery.
Clearly, the evolution of the pacemaker did not cease with the advent of monitoring capability. Indeed, the complexity of pacemakers has continued to increase in order to address the physiological needs of patients as well as the efficiency, longevity, and reliability of the pacemaker. For instance, even the early demand pacemakers provided stimulation pulses, when needed, at a fixed rate, such as 70 pulses per minute. To provide a more physiological response, pacemakers having a programmably selectable rate were developed. So long as the heart was beating above this programmably selected rate, the pacemaker did not provide any stimulation pulses. However, if the heart rate fell below this programmably selected rate, the pacemaker sensed the condition and provided stimulation pulses as appropriate.
Another major step in adding complexity and functionality to pacemakers occurred with the advent of pacemakers that had dual chamber capability. Dual chamber pacemakers are capable of sensing and/or pacing in two chambers, typically the right atrium and right ventricle. Accordingly, the distal ends of an atrial lead and a ventricular lead are coupled to the dual chamber pacemaker. The proximal end of the atrial lead is threaded through the pulmonary vein and into the right atrium of the heart. Similarly, the proximal end of the ventricular lead is threaded through the pulmonary vein, through the right atrium, and into the right ventricle of the heart. Each lead includes a mechanism on its proximal end that attaches to the inner wall of the heart to establish the required electrical connection between the pacemaker and the heart. Dual chamber pacemakers, as compared to single chamber pacemakers, typically function in a more physiologically correct manner.
To provide even further physiological accuracy, pacemakers have now been developed that automatically change the rate at which the pacemaker provides stimulation pulses. These pacemakers are commonly referred to as xe2x80x9crate-responsivexe2x80x9d pacemakers. Rate-responsive pacemakers sense a physiological parameter of the patient and alter the rate at which the stimulation pulses are provided to the heart. Typically, this monitored physiological parameter relates to the changing physiological needs of the patient. For instance, when a person is at rest, the person""s heart need only beat relatively slowly to accommodate the person""s physiological needs. Conversely, when a person is exercising, the person""s heart tends to beat rather quickly to accommodate the person""s heightened physiological needs.
Unfortunately, the heart of a person in need of a pacemaker may not be able to beat faster on its own. Prior to the development of rate-responsive pacemakers, patients were typically advised to avoid undue exercise, and pacemaker patients that engaged in exercise tended to tire quickly. Rate-responsive pacemakers help relieve this constraint by sensing one or more physiological parameters of a patient that indicates whether the heart should be beating slower or faster. If the pacemaker determines that the heart should be beating faster, the pacemaker adjusts its base rate upward to provide a faster pacing rate if the patient""s heart is unable to beat faster on its own. Similarly, if the pacemaker determines that the patient""s heart should be beating more slowly, the pacemaker adjusts its base rate downward to conserve energy and to conform the patient""s heartbeat with the patient""s less active state.
As noted above, pacemakers have historically been employed primarily for the treatment of heart rates which are unusually slow, i.e., bradyarrhythmias. However, over the past several years cardiac pacing has found significantly increasing usage in the management of heart rates which are unusually fast, i.e., tachyarrhythmias. Anti-tachyarrhythmia pacemakers take advantage of the previously mentioned inhibitory mechanism that acts on the secondary natural pacemakers to prevent their spontaneous rhymicity, sometimes termed xe2x80x9cpostdrive inhibitionxe2x80x9d or xe2x80x9coverdrive inhibitionxe2x80x9d. In essence, the heart may be stimulated with a faster than normal pacing rate (1) to suppress premature atrial or ventricular contractions that might otherwise initiate ventricular tachycardia, flutter (a tachyarrhythmia exceeding 200 bpm), or fibrillation or (2) to terminate an existing tachyarrhythmia.
Typically, these pulses need only be of sufficient magnitude to stimulate the excitable myocardial tissue in the immediate vicinity of the pacing electrode. However, another technique for terminating tachyarrhythmias, referred to as cardioversion, utilizes apparatus to shock the heart synchronized to the tachyarrhythmia with one or more current or voltage pulses of considerably higher energy content than that of the pacing pulses. Defibrillation, a related technique, also involves applying one or more high energy xe2x80x9ccountershocksxe2x80x9d to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections to allow reestablishment of an organized spreading of action potential from cell to cell of the myocardium and, thus, restore the synchronized contraction of the mass of tissue.
In the great majority of cases, atrial fibrillation is hemodynamically tolerated and not life-threatening because the atria provide only a relatively small portion (typically on the order of 15 to 20 percent) of the total volume of blood pumped by the heart per unit time, typically referred to as cardiac output. During atrial fibrillation, the atrial tissue remains healthy because it is continuing to receive a fresh supply of oxygenated blood as a result of the continued pumping action of the ventricles. Atrial tachyarrhythmia may also be hemodynamically tolerated because of the natural protective property of the junctional tissue attributable to its longer refractory period and slower conductivity than atrial tissue. This property renders the junctional tissue unable to respond fully to the more rapid atrial contractions. As a result, the ventricle may miss every other, or perhaps two of every three, contractions in the high rate atrial sequence, resulting in 2:1 or 3:1 A-V conduction and, thus, maintain relatively strong cardiac output and an almost normal rhythm.
Nevertheless, in cases where the patient is symptomatic or at high risk in events of atrial tachyarrhythmia or fibrillation, special treatment of these atrial disorders may be appropriate. Such circumstances may include, for example, instances where the patient suffers from ventricular heart disease and cannot easily withstand even the small consequent reduction of ventricular pumping capability, as well as instances where the rapid atrial rhythm is responsible for an excessively rapid ventricular rate. The methods of treatment commonly prescribed by physicians for treating atrial tachyarrhythmia and fibrillation include medication, catheter ablation, pacing therapy, cardiac shock therapy, and in some cases, surgically creating an A-V block and implanting a ventricular pacemaker.
In contrast to the atrial arrhythmias discussed above, cardiac output may be considerably diminished during an episode of ventricular tachyarrhythmia because the main pumping chambers of the heart, the ventricles, are only partially filled between the rapid contractions of those chambers. Moreover, ventricular tachyarrhythmia can present a risk of acceleration of the arrhythmia into ventricular fibrillation. As in the case atrial fibrillation, ventricular fibrillation is characterized by rapid, chaotic electrical and mechanical activity of the excitable myocardial tissue. However, in contrast to atrial fibrillation, ventricular fibrillation manifests an instantaneous cessation of cardiac output as the result of the ineffectual quivering of the ventriclesxe2x80x94a condition that typically requires almost immediate treatment.
Conventional cardiac stimulators monitor the ventricular rate to determine the nature of an arrhythmia. When a ventricular tachyarrhythmia is detected, the cardiac stimulator delivers anti-tachyarrhythmia pacing therapy to the ventricle or a higher level shock to the ventricle.
More recently, there has been a combination of certain complementary technologies, namely the combination of anti-tachycardia pacemakers with dual-chamber rate-responsive pacemakers. Generally speaking, a dual-chamber rate-responsive anti-tachycardia pacemaker offers improved performance over the pacemakers discussed above. However, pacemakers of this type exhibit certain disadvantages which are described below along with certain exemplary methods and apparatus directed to addressing these disadvantages.
Certain aspects commensurate in scope with the disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.
During the operation of a cardiac stimulator, it is generally desirable that the pacing not delay the detection of a tachyarrhythmia In conventional cardiac stimulators, such as some of those described above, this function is accomplished by providing discreet rate zones for tachyarrhythmia detection and bradyarrhythmia treatment, where the fastest demand pacing rate is less than the slowest tachyarrhythmia detection rate. As a result, the shortest pacing interval will always be longer than the longest tachyarrhythmia interval. Thus, this ensures that a detected ventricular event will not be followed by a demand pace, which may obscure a tachyarrhythmia event, until the longest possible interval for tachyarrhythmia detection has expired. While this functionality works well for cardiac stimulators which pace in the ventricle only, it has been discovered that such functionality could obscure certain tachyarrhythmia events in dual-chambered cardiac stimulators, primarily because the atrial pacing may also obscure the detection of tachyarrhythmia events.
As described in detail below with respect to the disclosed embodiments, in a conventional dual-chambered cardiac stimulator using such conventional functionality, there is a region where a ventricular tachyarrhythmia may exceed the tachyarrhythmia detection rate boundary in a region where the VA interval is shorter than the VT interval. Thus, detection of the ventricular tachyarrhythmia in this region causes the VA interval to be restarted so that an atrial demand pace would be delivered at the end of the VA interval. Therefore, there is a possibility that this atrial demand pace will obscure the subsequent ventricular tachyarrhythmia sensing and, thus, potentially delay detection of a ventricular tachyarrhythmia.
To address this situation, a number of techniques are described in detail below. In accordance with one technique, it is determined whether a ventricular event should be classified as a ventricular tachyarrhythmia. If not, the VA interval is restarted as usual. However, if the ventricular event may be classified as a ventricular tachyarrhythmia, it is determined whether the ventricular event falls within the region in which an atrial pacing event may obscure its detection. If not, then the VA interval is restarted as usual. However, if the ventricular event falls within this region, the VA interval is restarted with the VT rate detection boundary. This has the effect of lengthening the VA interval and the AA interval in this region so that atrial pacing events will not obscure the sensing and treatment of ventricular tachyarrhythmias in the region.
This technique may be modified by using a tachycardia rate different than the ventricular tachycardia rate detection boundary. For example, a tachycardia rate may be selected between well tolerated tachyarrhythmias and moderately tolerated tachyarrhythmias so that the VA interval and the AA interval are lengthened only when ventricular events fall within an upper portion of the previously discussed region. Although the well tolerated ventricular tachyarrhythmias may be obscured by atrial pacing events using this technique, the more clinically significant tachyarrhythmias will not be obscured.
In another modification of the previously discussed technique, it is determined whether a ventricular event should be classified as a ventricular tachyarrhytia If not, the VA interval is restarted as usual. However, if the ventricular event may be classified as a ventricular tachyarrhythmia, the VA interval is restarted with the VT rate detection boundary, and the VA interval is extended so that the AA interval does not exceed the maximum pacing rate. This technique has the effect of lengthening the AA interval, and thus lowering the pacing rate, only to the extent required to prevent an atrial pacing event from obscuring a ventricular tachyarrhythmia.
Finally, the programmable ranges of various parameters may be restricted to reduce or eliminate the circumstance in which an atrial pacing event may obscure a ventricular tachyarrhythmia. For instance, the VT rate detection boundary may be set higher than the VA interval so that ventricular events classified as ventricular tachyarrhythmias are always faster Man the VA interval. Also, the maximum pacing rate may be reduced so that the resulting VA interval is raised above the VT interval in the region above the VT rate detection boundary. Further, the AV interval may be reduced in the region to effectively raise the VA interval above the VT interval in the region above the VT rate detection boundary. Indeed, various combinations of these three restriction techniques may be used to program the cardiac stimulator to best fit a particular patient""s needs while minimizing the region in which ventricular tachyarrhythmias may be obscured by an atrial pacing event.
The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:
FIG. 1 illustrates a cardiac stimulator having two leads coupled to a patient""s heart;
FIG. 2 illustrates a block diagram of an exemplary cardiac stimulator;
FIG. 3 illustrates a diagram of a typical heart rate spectrum that illustrates programmable rates at the boundaries of each arrhythmia class;
FIG. 4 illustrates a timing diagram of exemplary atrial and ventricular events;
FIG. 5 illustrates a graphical representation of basic time intervals versus rate occurring within a conventional dual-chamber cardiac stimulator;
FIG. 6 illustrates a graphical representation of basic time intervals versus rate for a dual-chamber cardiac stimulator in accordance with the present invention;
FIG. 7 illustrates a flow chart depicting the functioning of a dual-chamber cardiac stimulator in accordance with FIG. 6;
FIG. 8 illustrates a graphical representation of basic time intervals versus rate for an alternate embodiment of FIG. 7;
FIG. 9 illustrates a graphical representation of basic time intervals versus rate for an alternate embodiment of a dual-chamber cardiac stimulator in accordance with the present invention;
FIG. 10 illustrates a flow chart depicting the functioning of a cardiac stimulator in accordance with FIG. 9;
FIG. 11 illustrates a graphical representation of basic time intervals versus rate for an alternate embodiment of a dual-chamber cardiac stimulator in accordance with the present invention, where the VT boundary rate is set higher than the intersection of the VT and VA curves;
FIG. 12 illustrates a graphical representation of basic time intervals versus rate for an alternate embodiment of a dual-chamber cardiac stimulator, where the maximum pacing rate is set low enough so that the resulting VA curve is raised above the VT curve in the region above the VT boundary; and
FIG. 13 illustrates a graphical representation of basic time intervals versus rate for an alternate embodiment of a dual-chamber cardiac stimulator in accordance with the present invention, where the interval is reduced so that the resulting VA curve is raised above the VT curve in the region above the VT boundary.